searching for long duration aftershocks in continental interiors miguel merino, seth stein...
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Searching for Long Duration Aftershocks in Continental Interiors
Miguel Merino, Seth SteinNorthwestern University
Mid-continental seismicity is time-variable
Faults switch on & off: mechanisms unclear
Active for short periods & dormant for long ones
Aftershocks continue for long times
What does a seismicity map tell us?
McKenna. Stein & Stein, 2007
“During the past 700 years, destructive earthquakes generally occurred in different locations, indicating a migration of seismicity with time.”
(Camelbeeck et al., 2007)
Royal Observatory of Belgium Catalog
Migrating seismicity:
NW Europe
during the periodprior to the periodinstrumental events
Earthquakes in North ChinaEarthquakes in North China
OrdosPlateau
Shan
xi Gr
aben
Bohai Bay
Beijing
1303 HongtongM 8.0
Liu, Stein & Wang 2010
Weihi rift
during the periodprior to the periodinstrumental events
Earthquakes in North ChinaEarthquakes in North China
OrdosPlateau
Shan
xi Gr
aben
Bohai Bay
Beijing
1556 HuaxianM 8.3
Weihi rift
during the periodprior to the periodinstrumental events
Earthquakes in North ChinaEarthquakes in North China
OrdosPlateau
Shan
xi Gr
aben
Bohai Bay
Beijing
1668 TanchengM 8.5
Weihi rift
during the periodprior to the periodinstrumental events
Earthquakes in North ChinaEarthquakes in North China
OrdosPlateau
Shan
xi Gr
aben
Bohai Bay
Beijing
1679 SanheM 8.0
Weihi rift
during the periodprior to the periodinstrumental events
Earthquakes in North ChinaEarthquakes in North China
OrdosPlateau
Shan
xi Gr
aben
Bohai Bay
Beijing
1966 XingtaiM 7.2
1976 TangshanM 7.8
1975 HaichengM 7.3
Weihi rift
Historical
Instrumental
Shan
xi Gra
ben
Weihi rift
Rate-state friction predicts aftershock duration
1/loading rate
Plate boundary faults quickly reloaded by steady
plate motion after large earthquake
Faults in continents reloaded much more
slowly, so aftershocks continue much longer
Current seismicity largely aftershocks rather than
implying location of future large events
Stein & Liu, 2009
General pattern of long aftershock sequences in slowly deforming continental interiors
Stein & Liu 2009
Long duration aftershock sequences resolvable from low intraplate
background
Parsons, 2009
California Seismicity 2003-2006
Many aftershock
zones are still visible
today, including 1952
Kern County earthquake aftershocks
Seismicity (1970-1974) visible in the aftershock zones of large past central Nevada seismic belt
earthquakes
Systematicdecrease in
seismicity withtime
Ryall, 1977
Aftershock sequences continue in Haicheng and Tangshan >30 years after the main shocks
M. Liu
Question:
Do zones of low-magnitude seismicity within continents reflect aftershocks continuing for long times
or loci of future earthquakes
What does a seismicity map tell us?
McKenna. Stein & Stein, 2007
Tuttle (2009)
Meers fault, OklahomaActive 1000 years ago, dead now
Obermeier, (1998)
Wabash: M~7 6 Kybp
Seismicity migrates in Central US
Is seismicity migrating from New Madrid to Wabash?
What does seismicity show?
Why b-value difference?
1) Wabash has a relatively low b value. Could indicate high fault stressing rates, consistent
with stress migration following large 1811-1812 earthquakes
2) New Madrid has a relatively high b value. Could reflect NMSZ having more small earthquakes
that are 1811-1812 aftershocks
Li et al., 2007
High stressing rate could give rise to low b value
Wiemer & Schorlemmer. 2007
San Andreas Fault, Parkfield
2) Many recent NMSZ
events appear to be 1811-12 aftershocks
- have been used to map presumed rupture
- rate & size decreasing
- largest at the ends of presumed 1811-12
ruptures
Stein & Newman, 2004
To see whether New Madrid or Wabash anomalous,compare to central U.S background seismicity
NM
W
Although we often consider b=1 the norm, low values are common for intraplate areas
Sykes et al. 2008
Okal & Sweet 2007
Okal and Romanowicz, 1994
Numerical Simulation:How long do we expect to see
aftershocks in New Madrid• ++
Aftershock catalog:- Omori’s Law for #
earthquakes per year- b value (probability of given earthquake M/yr)-Uniformly distributed
NS and normal distribution EW across a
NS fault
Aftershock catalog:- Omori’s Law for #
earthquakes per year- b value (probability of given earthquake M/yr)-Uniformly distributed
NS and normal distribution EW across a
NS fault
Background catalog:- a value/unit area
- b value-Uniformly distributed
in model region
Background catalog:- a value/unit area
- b value-Uniformly distributed
in model regionCombine
background and aftershock catalogs
for designated aftershock region
Combine background and
aftershock catalogs for designated
aftershock region
T-test to check how long
aftershocks are detectable
from the background
T-test to check how long
aftershocks are detectable
from the background
Synthetic Catalogs
Synthetic Aftershock Catalog
Synthetic Background Catalog
Synthetic Catalog T-Test
€
TValue =XAftershock−μBackgroundSdBackground / n
Test probability that observed rate of seismicity (aftershocks) is significantly different from mean (background)
Different length catalogs simulated to decide when aftershocks can no longer be resolved from background
Conclusions
New Madrid seismicity dominated by aftershocks of 1811-1812 earthquakes
Seismicity here would remain detectably different from the background for ~200-230 years
Aftershocks could still be noticeable for even longer time in less seismic areas (US east coast?)
Concentrations of small intraplate seismicity may reflect large past (“ghost”) earthquakes
Could test possibility with paleoseismology